Ancient rivers in Eastern Washington: a study of the 15-million-year-old Clearwater-Salmon River

By Matthew Morriss
Whitman College, Walla Walla, Washington

Several drainages originating on the periphery of the Columbia Plateau have fluvial histories going back to the Miocene (23-5.3 Ma).  Numerous flood basalts inundated these paleodrainages, moving river channels, and in some cases, altering the river’s entire course.  In the Miocene, the Clearwater-Salmon (CWS) River drained the Clearwater range in North-Central Idaho.  Rocks present in this mountain range represent nearly 1 billion years of sedimentation, metamorphism, and igneous intrusion.  Subsequent to channel incision on the Columbia Plateau, the Columbia River Basalts filled in the Clearwater-Salmon’s channel, leaving an identifiable interbed of sediment where the river once flowed.  In this study, three outcroppings of CWS cobbles were studied stratigraphically, sedimentologically, petrologically, and geochemically.  These techniques were employed in order to further understand the course taken by the CWS River, and the changes through time to the provenance area.  This project was supported by the Abshire Research Scholar Award.  Faculty Sponsor: Patrick Spencer.